This invention relates to a valve timing control apparatus for use in an internal combustion engine to provide desired valve timing(s) according to engine operating conditions.
For example, U.S. Pat. No. 4,231,330 discloses a valve timing control apparatus for controlling the intake or exhaust valve timing according to engine operating conditions. The valve timing control apparatus employs a cylindrical gear having internal and external threaded portions, one of which is in the form of a helical gear. The external threaded portion engages with the inner threaded portion of a rotary drum which is formed on its outer peripheral surface with sprocket teeth for engagement with a timing chain engaging with a sprocket mounted on the crankshaft of the engine. The internal threaded portion of the cylindrical gear engages with the external threaded portion formed on the camshaft of the engine. The camshaft is rotated at an angle with respect to the rotary drum by applying fluid pressures to move the cylindrical gear in an axial direction of the camshaft.
However, the conventional valve timing control apparatus employs an expensive helical gear. The helical gear is required to be machined with high accuracy sufficient to provide smooth valve timing control. Additionally, the conventional valve timing control apparatus is arranged to rotate the camshaft at a great angle with respect to the sprocket by moving the cylindrical gear, which extends axially of the camshaft, to a great distance. This arrangement requires a large space for the valve timing control unit, resulting in a large-sized engine. Still further, the conventional valve timing control apparatus utilizes oil pressure to control the position of the cylindrical gear. However, it is very difficult to maintain the oil pressure at a constant since the oil viscosity varies with changes in oil temperature and engine speed.